btrfs: fix error pointer dereference after failure to allocate fs devices

At device_list_add() we allocate a btrfs_fs_devices structure and then
before checking if the allocation failed (pointer is ERR_PTR(-ENOMEM)),
we dereference the error pointer in a memcpy() argument if the feature
BTRFS_FEATURE_INCOMPAT_METADATA_UUID is enabled.
Fix this by checking for an allocation error before trying the memcpy().

Fixes: f7361d8c ("btrfs: sipmlify uuid parameters of alloc_fs_devices()")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -746,13 +746,13 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 
 	if (!fs_devices) {
 		fs_devices = alloc_fs_devices(disk_super->fsid);
+		if (IS_ERR(fs_devices))
+			return ERR_CAST(fs_devices);
+
 		if (has_metadata_uuid)
 			memcpy(fs_devices->metadata_uuid,
 			       disk_super->metadata_uuid, BTRFS_FSID_SIZE);
 
-		if (IS_ERR(fs_devices))
-			return ERR_CAST(fs_devices);
-
 		if (same_fsid_diff_dev) {
 			generate_random_uuid(fs_devices->fsid);
 			fs_devices->temp_fsid = true;